To date there has been a large amount of research and development by researchers and industry relating to thermoplastic resin films. Thermoplastic resin films have excellent mechanical properties, chemical resistance, transparency, and water vapor permeability while the raw materials are relatively inexpensive. Therefore, the films are used in a wide range of fields including packaging, miscellaneous goods, agriculture, industry, food, and medicine.
In recent years, there have been a large number of instances in which thermoplastic resin films are used in optics. Examples of thermoplastic resins include polycarbonates, cyclic polyolefins, polyethylenes, and polypropylenes. In particular, polycarbonates, cyclic polyolefins, and the like have relatively good optical transparency and are especially advantageously used in diffusion sheets, prism sheets, antireflective films, original phase difference films, and other undrawn products. Also the films can be endowed with optical anisotropy (orientation) when subjected to a drawing process (uniaxial or biaxial drawing). Films fabricated from a thermoplastic resin endowed with such an orientation can be advantageously used as phase difference films in liquid crystal displays (LCDs) and the like.
To date, various methods for producing such thermoplastic resins are known and have been put into practice. Examples of methods that are generally used to manufacture thermoplastic resin films on an industrial basis include solution casting in which a resin solution obtained by dissolving resin in a solvent is cast on a glass plate or the like to form a film, a T-die extrusion method in which molten resin is extruded through an extruder and then cooled over a chilled roll, a tube extrusion method in which molten resin is extruded in tubular form through an extruder, and an inflation extrusion method in which resin extruded in the form of a tube is molded while air pressure is applied to the interior of the tubular resin.
The film width of the thermoplastic resin films described above is determined by the width of the dies used in T-die extrusion method and by the width of the casting dies used in the solution casting method. It is difficult to match a single die to products having differing film widths, and unusable excess is produced in cases where the width of the die and the width of the product differ. The film width can be forcibly narrowed using the Deckle method, but there is a problem in that gel caused by the retention of resin is more readily produced. In the inflation extrusion method the film width can be changed while using a single annular die by changing the blow ratio, but the same quality film cannot necessarily be obtained because drawing state changes depending on the blow ratio. In the tube extrusion method, the tube diameter can be changed by varying the diameter of the sizing pipe, but the same quality is not always obtained and it is generally difficult to adjust the film width while maintaining the same quality because the drawing conditions are different.
Since the rate of cooling is ordinarily faster at the two ends of a film than at the center of a film, the quality of long films manufactured by the T-die extrusion method varies from the two ends of the film to the center. For this reason, a product that encompasses the entire width cannot be achieved, and the two ends of the film (the ear portions) are removed. Also, in cases where the thickness in the TD direction is not uniform, film bumps are readily generated when the film is wound, and the winding axis is sometimes moved back and forth (oscillated) in the axial direction in order to avoid this problem. In this case as well, the product yield is reduced because the two ends of the film must be cut away in order to align the width of the film. In relation to tubular films manufactured by the tube extrusion method and the inflation extrusion method, the ends of the film are ordinarily removed after the film has been folded and dismounted when the application is not a bag shape. Therefore, a product that encompasses the entire width cannot be achieved and yield is predictably reduced.
However, to manufacture a film such as a phase difference film in which the resin is oriented at a particular angle, a long film manufactured by the T-die method, solution cast method, tube extrusion method, inflation method, or the like is drawn in the TD and/or MD direction, and is cut so that the orientation of the resin matches the desired angle. In this case, unusable portions were generated at the two ends of the long film. In view of the above, a method of drawing the long film diagonally in T-die extrusion has been proposed with the intent of reducing the unneeded portions (e.g., see patent documents 1 and 2). With this method, the two ends of the film in the width direction are clamped using a jig and the film is conveyed. In this case, the film is conveyed so that the conveyance velocity in one direction of the width direction of the film is faster than that in the other direction, whereby the film is stretched in a diagonal fashion in temperature conditions in which the molecular orientation of the film resin can be changed.
As a method of manufacturing films of a resin at a certain angle, a method is proposed wherein the thermoplastic tubular resin film has been wound once around a roller or the like without cutting, the resin is thereafter unwound while being rotated, an insertion member or the like is inserted into the tubular resin film, and the film is cut and removed by use of fixed cutting while the tubular shape is restored (e.g., see patent documents 3, 4, and 5).
Also, a method of manufacturing a diagonally oriented thermoplastic resin film is proposed in which an annular die is rotated, whereby the thermoplastic resin film is caused to rotate while moving forward, the film is subsequently drawn along a single axis in the direction of progression, and the rotating tubular resin film is thereafter vertically bisected by a stationary knife (e.g., see patent document 6).
Another method of manufacturing diagonally oriented thermoplastic tubular resins is proposed in which the tubular resin film is extruded from a circular die, is thereafter momentarily cooled, and is then reheated, whereupon the tubular resin film is twisted while the tubular shape is maintained (e.g., see patent document 7). In the patent document mentioned above, a tubular die is fixed and a mandrel provided with suction slits is disposed on the inner side of the tubular resin film, the tubular resin film is brought into contact with the mandrel by suctioning, and the mandrel is rotated to thereby twist the tubular resin film.
Recently, it has been proposed that a resin film product can be fabricated in which the nonuniformity of thickness of the tubular resin film is reduced and which is provided with a small, uniform, flat surface by using stabilizing means, and that a high quality resin film product endowed with an orientation and having few thickness and drawing nonuniformities can be fabricated by passing the film through a drawing unit and holding unit made of a porous mandrel or the like and drawing the film (e.g., see patent documents 8 and 9). A tubular resin film manufactured by this method entails rotating the cutting device about the rotational axis in the lengthwise (MD) direction of the tubular resin film. A film of arbitrary width can be manufactured by adjusting the rate of rotation. Since the tubular resin film can be oriented at the same time in the MD direction or in the circumferential (TD) direction, a film having an arbitrary orientation can be obtained when the film is cut using the above mentioned cutting device. Similarly, a method of manufacturing a film having a diagonal orientation is known in which the cutting device rotates about the rotational axis in the lengthwise (MD) direction of the tubular resin film (e.g., see patent documents 10 and 11).
As described above, it is difficult to arbitrarily adjust the film width using an ordinary film manufacturing method. The unusable portion of the film greatly increases and it is necessary to recover and remanufacture the film, especially when a resin film is manufactured having an arbitrary orientation such as a phase difference film. On the other hand, there are various problems as described above when a film having an arbitrary orientation is manufactured.
In the T-die extrusion method, a method has been proposed in which the film is stretched diagonally, but since the transport rates of the two sides of the long film are different, it is difficult to control the drawing conditions and a complicated apparatus is used in achieving a uniform thickness and a uniform phase difference across the entire film surface. Also, since the ends of the film must be clamped in a jig, the clamped portion cannot be used as a product due to film damage.
In the method in which the tubular resin film is folded and wound around a roller or the like in a single process without being cut, the resin is thereafter unwound while being rotated, an insertion member or the like is inserted into the tubular resin film, and the film is cut and wound using fixed cutting means while the shape is restored to a tubular shape, a high-quality film cannot be produced because folding wrinkles are generated by folding the film in a single process. Also, wrinkles readily occur if the film is thin because the inserted member is moved while the tubular film is being rotated.
In the method in which the thermoplastic resin film is moved forward while being rotated due to the rotation of the annular die, and the rotating tubular resin film is drawn along a single axis in the direction of progression and is vertically cut by a stationary knife, folding wrinkles do not occur because the film is never folded. This method is characterized by rotating the annular die in order to cut the tubular film into a helical shape. However, the melting viscosity of the resin is high the moment it is extruded from the annular die, and it is therefore very easily affected by vibrations, nonuniformities in stress, nonuniformities in temperature, and nonuniformities in air quantity. In other words, when the annular die rotates, an external force is generated in the direction of rotation the moment the resin is extruded from the annular die such that the uniformity of the resin film thickness and the surface appearance readily deteriorate, and it becomes difficult to produce a high-quality resin film. Also, wrinkles are easily generated if the film is thin because the interior of the mandrel moves while the tubular film is rotated.
In the method of manufacturing a diagonally oriented thermoplastic tubular resin in which the tubular resin film is extruded from the annular die, is thereafter momentarily cooled, and is then reheated, whereupon the tubular resin film is twisted while the tubular shape is maintained, there is a possibility that the film surface will be damaged when the tubular resin film is brought into contact with the mandrel by suction, and there is a possibility of wrinkles forming in the film when the tubular resin film is heated and twisted in a softened state. As expected, it is difficult to obtain a high quality tubular film in this method as well because it is difficult to control the drawing conditions in order to achieve a uniform thickness and a uniform phase difference across the inside plane of the film.
On the other hand, with the method in which the cutting device rotates about the lengthwise (MD) direction of the tubular resin film in the manner shown in patent documents 10 and 11, the cutting and the weight of the winding device as such increases when the film being handled becomes wider and the line speed is accelerated, and considerable centrifugal force may be applied to the cutting and to the winding device. Therefore, the apparatus and the support body for supporting these become bulky. Also, because the rotational angular velocity of the cutting device increases together with the increase in line speed, the cutting device is made bulky so as to withstand the centrifugal force. It is very likely that equipment costs would increase because a large installation location must be used. Furthermore, in cases where the rotation speed has been increased, the operation of first winding the film onto the roller and the operation of exchanging film rolls become difficult, and there are areas of uncertainty in term of safety.    [Patent Document 1] Japanese Laid-open Patent Application No. 4-164626    [Patent Document 1A] Japanese Laid-open Patent Application No. 2004-20701    [Patent Document 3] Japanese Laid-open Patent Application No. 10-226023    [Patent Document 4] Japanese Laid-open Patent Application No. 51-55368    [Patent Document 5] Japanese Laid-open Patent Application No. 55-59407    [Patent Document 6] Japanese Patent Publication (Kokoku) No. 40-5319    [Patent Document 7] Japanese Laid-open Patent Application No. 47-34656    [Patent Document 8] WO2004-067260    [Patent Document 9] WO2004-067266    [Patent Document 10] Japanese Laid-open Patent Application No. 2003-215320    [Patent Document 11] Japanese Laid-open Patent Application No. 55-161619